Step and repeat lithography devices, called scanners or wafer steppers, are commonly used to mass produce semiconductor devices such as integrated circuits (ICs). Typically, a light source and various lenses are used to project an image of a mask onto a photosensitive coating of a semiconductor wafer. The projected image of the mask imparts a corresponding pattern on the photosensitive coating. This pattern may be used to selectively etch or deposit material to form desired semiconductor devices.
A photolithographic exposure system includes a light source, a patterned mask, and an optic system that is used to focus and project an image of the mask onto a target, such as a wafer. Several mechanical parts are used to move the wafer to various positions as a new portion of the wafer is exposed to the pattern of the mask with the optic system and the light source. During the manufacturing process, in some instances, the distance between the target and the exposure system may change. In other instances, the focal length of the optical source may change. In either instance, the previously in-focus image at the target may become out of focus. When out of focus, or defocussed, the photolithographic process may produce semiconductor devices that fail or are substandard. For example, critical dimensions may vary outside a selected variance. Accordingly, it is desirable to determine when the photolithographic exposure system is defocussed so that the source of the problem can be both detected and corrected.
Furthermore, continuously reducing the feature size of semiconductor devices seems to be a constant industry goal. For example, reducing the feature size in solid-state memory architecture can be an effective way to increase the capacity of such memories for a given amount of circuit area However, as the feature size continues to become smaller, the problem associated with defocussing of the photolithographic equipment increases, since the critical dimension of certain features has a smaller tolerance range.